Sound masking

Building interiors br Sound masking can be used anywhere to ensure speech privacy or reduce distractions Sound masking is typically used in selected workspaces but it can also be helpful in residential environments The most common sound masking installations are br Open office plans open offices can be either too quiet where someone dropping a pen in the next cubicle is distracting or too noisy where the conversations of others in the office make it impossible to concentrate Open offices can benefit from sound masking because the added sound covers existing sounds in the area making workers less distracted and more productive br Private offices private offices and other enclosed spaces often appear to provide privacy but actually do not Many times walls are lightweight and do not extend to the ceiling deck only to the ceiling tile In these cases sound can easily travel through partitions or over the walls Sound masking can be provided in adjacent private offices or in hallways outside of private offices to ensure that confidential conversations remain confidential br Public spaces sound masking is useful for reception areas pharmacies waiting rooms and financial institutions Sound masking is provided in the area where conversations should not be heard not necessarily in the area where the conversation is taking place For instance a psychiatrist does not want those in the waiting room to overhear a private conversation with a patient so sound masking is provided in the waiting area not in the psychiatrist s office br Sound masking may also be used to hide other unwanted noise such as the intermittent sounds from machinery In an office this could be sound of elevators and compressors Sound masking may render conversations unintelligible by nearby listeners and may thus help compliance with HIPAA and GLBA regulations br Sound masking is being used to protect confidential privacy in areas where sensitive or classified conversations are being held The applications among others are in government military military contractors corporate board rooms and legal offices The requirements for this type of masking are more stringent The sound must be guaranteed to be continuous during room use performance must be verified and the equipment must be able to protect windows doors walls and ducts with vibration maskers instead of loudspeakers br Exteriors br A number of cases exist where sound masking has been successfully installed for exterior applications the most common target of concern being roadway noise In one example application a large artificial waterfall was constructed as part of the garden exterior of an urban hotel in Santa Rosa California The waterfall cascades down an extensive wall approximately four meters in height and functions both for sound masking and as a physical barrier to road noise br Sound masking systems br In plenum br The plenum is the space between a ropped ceiling and the upper deck for the floor In Plenum sound masking systems which employ a network of loud speakers located completely within the plenum were the first such systems developed they have been in use since the 1960 Plenum based speakers typically range from 4 to 10 inches in diameter and generally face upwards towards the upper deck This is done to reflect sound from the speakers to broaden as much as possible the footprint from the speaker in the work area br The actual pattern of the received acoustic energy in the workspace from speakers in the plenum is complicated by a number of factors all of which cause spatial variability in the sound masking field in the workspace First because loud speakers actually radiate in all directions some energy at low frequencies is radiated downwards Thus some sound variability occurs directly below the speakers Second dropped ceilings have a wide range of acoustical transparency or transmission loss their degree of sound penetration directly to the space below Some common lightweight office ceilings tiles particularly those made of fiberglass have a high degree of transparency which increases sound variability below the speakers Third the plenum is acoustically complicated by the presence of HVAC ducts large beams and other structural members which act to ompartmentalize the masking sound and cause scattering and reflections This scattered sound can also cause spatial variability Fourth when less transparent acoustical tiles e g mineral fiber tiles are used a reverberant acoustic build up occurs in the plenum that can cause significant overflow from the intended treated space e g an open plan office into spaces where sound masking may not be needed or wanted e g private offices or conference rooms Finally when the plenum is used as a vehicle for return air for the HVAC system the ceiling necessarily has vents or open air returns If these returns are untreated they will act as direct transmitters of the acoustic field from the plenum to the office area and create additional variability br Treating the open air returns is straightforward but does add cost to the installation Properly tuned and adjusted plenum based systems when used in conjunction with treated open air returns have been shown to provide uniformity within many target sound masked spaces Uniformity can be achieved by adjusting the acoustic output of individual or small groups of speakers Adjustments routinely include changes in output volume and output spectra of individual speakers To provide this adjustment capability additional system electronics for individual speakers or for small groups of speakers are required br Direct field br Direct Field sound masking systems have been in use since the late 1990 The masking acoustics is called irect field because the sound from any specific masking speaker travels directly to a listener without interacting with any other reflecting or transmitting feature Initially used as an accessory for open office cubicles direct field systems have been fully integrated into at least one open office furniture system and have been designed to be installed both in dropped ceilings and in offices without any absorptive ceiling systems When installed in dropped ceilings direct field systems use speakers that are mounted facing down When a ceiling tile is not available they are mounted facing down on any available structure sending the masking noise directly into the intended space Direct field masking requires speakers that are in effect omni directional meaning that they transmit energy equally in essentially all directions The use of omni directional speakers spaced appropriately for the work area typically on a grid equal to the ceiling height provides sound masking that is evenly distributed Using direct field speakers eliminates issues of spatial uniformity and verflow due to plenum conditions and open air returns Because the plenum and ceiling materials are not acoustically involved individual speakers do not have to be adjusted to counteract plenum conditions so no tuning is required and electronic complexity is minimized Also because the sound from direct field speakers does not have to interact with the ceiling tile the amount of energy required to produce a sound masked space is reduced br In Open Ceilings br In many installations particularly warehouses that have been converted to office space the masking speakers are hung in a similar manner to those above suspended ceilings Typically the speakers are mounted higher and the spacing between them is often closer The space generally has a very high structural ceiling so the sound created is quite diffuse occupants cannot locate the speakers easily by listening br Under Raised Floors br In offices that utilize raised floors masking speakers can be placed under them Special speakers have been designed to accommodate even very shallow cavities otherwise normal masking speakers can be used there Listener acceptance is very high with this type of design as the sound is very diffuse br Advances in Sound Masking br Sound masking has been in use for many years It is likely that Roman villas with interior fountains had the benefit of masking the sound of chariots Even today fountains are used in malls to provide humidity a pleasant environment and sound masking Due to increasing use of sound masking as a privacy tool electronic methods have had a number of improvements that enhance the performance and acceptability of them Equipment has been designed to protect sensitive conversations Masking has been applied successfully in hospitals and other medical facilities Masking speakers have been successfully located in open ceilings above suspended ceilings face down in suspended ceilings and under raised floors Most professionals have converged on a preferred spectrum and level of masking for both open and closed offices that balances the need for privacy with occupant acceptability Most modern equipment is now capable of handling these requirements Many systems have an initial ramp function This function permits the user to increase the masking level automatically to the desired level over many days Persons moving into a new office prefer slow changes and this function accommodates the acoustical aspect Some systems have a power ramp function When building power fails and then is turned on the sound masking level would jump up quite noticeably possibly creating a negative response This function acts similar to the initial ramp function but acts in minutes rather than days Some systems have a level scheduling function This permits the user to have the sound masking level vary each day based on office use If the office is fully occupied the masking is at its highest level As occupancy is low early in the workday and persons are preparing to leave in the afternoon the level can be reduced During night hours the level is set low so security guards can hear any suspicious sound as they tour the office Some systems have an adaptive function In this system a sound detector captures the sound separates the activity sound typically speech from the background sound masking and adjusts the masking so the difference between the two is kept constant Essentially the system applies sound masking only when it is needed br The Value of Sound Masking br Sound Masking is Dynamic It can be adjusted anytime and anywhere to handle changing levels of voices and other noises Walls and panels are static and cannot do this br Sound Masking Works at the Listeners Ear Voices travel on many paths from a speaker to a listener To block the paths between closed offices say the path must be defined or guessed and then protection added how much Because masking works at the listeners ear where all the paths come together it can be set to block all paths Since it can be measured accurately the desired protection can be done quickly br Sound Masking Systems are Audio Systems Since one design requirement is uniformity of the level it is advantegous to add paging and music as desired The sound is better and the cost addition is minimal br Sound Masking is Very Cost Effective The ratio of the amount of privacy afforded to its cost for a sound masking system is considerably better than that provided by walls or panels br Attributes of Successful Sound Masking Systems br Sound masking level For open offices depending on design the level can vary from 43 dB A to 48 dB A and for closed offices the masking levels can vary from none to 44 dB A Adjust of levels must be done in small increments of level near 1 dB br Sound masking spectrum contour The frequency distribution of sound masking levels at a minimum cover the frequency range of speech which is 160 to 8000 Hz Most experienced practitioners design a spectrum contour that decreases with frequency at about 4 dB per octave requiring more low frequency sound than high That particular shape of the contour has been found to be acceptable to occupants The level of that contour is set to provide the desired privacy so a balance between performance and acceptability can be achieved br Spatial uniformity Unlike most sound systems that call attention to themselves sound masking is designed to be background not recognized just as the background sounds outdoors are not recognized This is achieved by having sound masking uniform throughout a room Uniformity is best achieved by having an intervening material suspended ceiling raised floor between the masking speaker and the listener br Invisibility In keeping with the concept of being background it is desirable to have the sound masking system invisible The control equipment should be in a separate room Putting speakers behind an intervening material hides them br Sound diffusion When sound is diffuse arriving from all directions about equally most people are not even aware of it The quiet outdoor background is an example The sound from an airport is just the opposite the sound can be identified and disliked Diffusion is greatly improved by use of an intervening material br Applicability Masking systems need to accommodate the many locations in which masking speakers are put They include under raised floors above suspended ceilings in open ceiling plenums above discontinuous suspended ceiling tiles and on walls br Portability The system must be able to be moved to a new facility with minimum cost br Phasing When two masking speakers have the same signal but are separated from each other a person at the mid point between them can sense a wishing sound This is caused by level changes at various frequencies caused by phase shifts When strongly noticeable it is considered to be objectionable This effect is most pronounced in a commercial facility with persons moving about An intervening material can eliminate this effect If there is no intervening material special design is required to eliminate it br The Relationship of Sound Masking to Speech Privacy br Many times an owner has an office noise problem and has heard that sound masking wi

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